1. Field of the Invention
The present invention relates generally to apparatus and methods for filling voids with foamed cement grouts. More particularly, the present invention relates to an apparatus and method for the continuous mixing of foamed cement grout, and pumping this to a desired injection site.
2. Background Art
Foamed cement grouts have many applications in industry, such as for filling abandoned pipelines and other large voids in the earth, and for grouting of tunnel liners and similar structures. These grouts are formed by mixing a finished foam, which comprises a mass or aggregate of bubbles, with a cement slurry so that air spaces are entrained within the grout. Because of the relatively large volume of the entrained air, the amount of cement slurry which is needed to fill a particular cavity is greatly reduced from that which would be needed if an unfoamed slurry was used. This results in great cost savings, especially when filling very large voids. Other advantages of foamed cement grouts include the fact that, because they are fluid and non-shrinking, the need for contact grouting is eliminated.
Although foamed cement grout is thus a highly advantageous material for grouting, backfilling, void filling, and so forth, its success in many of these applications has been limited to a significant degree by the manner in which it is conventionally prepared. In short, to the best of Applicant's knowledge, foamed cement grouts have always been prepared for fill work by batch-type processes: the foam is typically mixed into the cement slurry in a tub or other vessel to form a batch of the grout, which is then pumped from this to the injection site.
This batch-process approach exhibits certain inherent inefficiencies and disadvantages, especially when it comes to large fill jobs. While it may be possible to prepare batches of foamed cement grout which are big enough to complete relatively small grouting jobs, this is simply not feasible in the case of larger jobs, such as the filling of abandoned pipelines, which may call for hundreds or thousands of cubic yards of grout. Obviously, the repeated starting and stopping which is involved in a batch process introduces a strong element of inefficiency on such projects, especially being that large crews of workers may be left standing idle between the injection of each batch.
Furthermore, the need to mix up separate batches of grout and inject these individually invariably leads to quality control difficulties. Apart from mix variations which occur inevitably from batch-to-batch, the batch-type processes are inherently incapable of permitting adjustment of the quality of the grout as it is being injected. For example, although the bubble structure of the fluid grout is very stable, and will withstand high pressures without loss of integrity, significant bubble loss may occur due to friction between the grout and the piping through which it is pumped. These friction losses are somewhat unpredictable, and naturally become more serious as pumping distances increase. The inability of the batch-type processes to adjust the quality of the grout (i.e., the foam content) to compensate for observed friction losses means that an entire batch of grout may be placed at the injection site with the foam structure being significantly deteriorated due to friction loss, resulting in a severe decrease in the volumetric yield of the batch.
Another serious problem which has been encountered with such conventional grouting systems, particularly when filling abandoned pipelines and other elongate voids, stems from their inability to deliver the grout continuously at a high volume rate over sustained periods. As was noted above, the bubble structure of the grout is very stable so long as there is free liquid in the mixture, and so this can be pumped at relatively high pressures. However, once hydration of the grout proceeds to the point where it takes an initial set, the bubble structure ceases to exist and is replaced by a simple void which is maintained only by the cement paste which surrounds it; if this is subjected to external pressure, as by the injection of additional grout adjacent to or on top of the first, the void structure is very easily collapsed, resulting in a severe loss of volumetric yield. Simply put, it is very difficult to avoid this when using a batch-type process, since the process is slow by its very nature and injection must periodically halt while another batch is being prepared, and the grout will continue to set up during these pauses; also the pumps and related equipment which are conventionally employed in these processes are not suited to high injection rates.
Accordingly, there exists a need for an apparatus and method for preparing foamed cement grout and pumping this to an injection site on a continuous basis, and not batch-wise. Also, there is a need for such an apparatus and method which will permit continuous monitoring and adjustment of the quality of the grout which is produced. Still further, there is a need for such an apparatus and method which will permit the grout to be injected at a sustained rate sufficiently high to avoid injection on top of previously-injected grout which has taken an initial set.
One specific application for foamed cement grout which was noted above is for the grouting or backfilling of tunnel liners. In most cases, a tunnel is not complete until a liner has been placed along the perimeter of the bored hole. In a typical technique, a tubular tunnel liner is placed within the cylindrical wall of the tunnel, which results in an annular cavity being formed between these.
It has been found advantageous to fill this cavity with foamed cement grout, but difficulties have been encountered when using this material in relatively long tunnels. The grout, once mixed, is usually relatively viscous, and tends to compress and cause friction and back-pressure when pumped through conduits. This difficulty becomes serious if it is necessary to pump the grout over great distances, as from the surface to an injection point far inside a tunnel. Attempts have been made to overcome these problems by mixing batches of foamed cement grout within the tunnel, as by transporting dry cement in bags to a small batch mixer inside the tunnel and then mixing this with water and foam to form the grout; however, this batch-type approach shares the disadvantages discussed above, being that it has proven exceedingly slow and expensive to practice, and it is very difficult to carry out with adequate quality control.
Accordingly, there exists a need for an apparatus and method for employing foamed cement grout to backfill tunnel liners which avoids the need to pump the grout over long distances into the tunnel bore. Furthermore, there is a need for such an apparatus and method for continuously forming and injecting such foamed cement grout within the tunnel, so that high volumes of grout can be placed over long distances quickly, and with a high degree of quality control.